Pacific Rim National Park Reserve of Canada

Red Tide Information

What Is Red Tide and How Does It Affect You?

What is red tide?

The world's water- both fresh and marine- contains a large variety of single-cell plants called microalgae. The individual plants are usually so small they require a microscope to be seen. When conditions are right, often in the summer in this area, one species of algae will multiply in great numbers. This is called a plankton bloom. Depending on the most common species of algae in the bloom the water colour may change. In this area blooms are often brown, red or blue. Many of the blooms in marine waters are red which has lead to the common term red tide.

How does it affect you?

Many of the red tide algae produce toxins. Different toxins are produced by different algae. Some toxins have a big impact on fish, others on mammals. The algae which is responsible for paralytic shellfish poisoning (PSP) is in the group of algae called Gonyaulax. These algae produce an extremely potent toxin, which affects nerve impulses and can lead to paralysis of muscles and may eventually end in asphyxiation. Warm-blooded animals are particularly sensitive to this toxin. Shellfish, such as clams, mussles, and oysters can feed on the algae with no ill effect. However, their tissue will accumulate the toxin when they feed on the algae. The problem for humans occurs when we eat shellfish that has accumulated large quantities of the toxin in their tissue. Shellfish containing the toxin do not appear different than shellfish that do not contain the toxin. In our area Fisheries and Oceans Canada regularly test shellfish for this toxin.

Fisheries and Oceans Canada base shellfish harvesting openings and closings on their rigorous monitoring procedures. If you are considering eating shellfish in a restaurant the shellfish should have been tested before going to market and should be safe to eat.

If you are considering harvesting shellfish, honour all Fisheries and Oceans Canada PSP notices and consumption closures. Warnings are posted for visitor safety, not for conservation purposes.